In recent years, there has been a widespread increase in efforts to implement wireless power transfer systems in both commercial and residential settings. Wireless power systems offer the promise of eliminating power/charging cords for a wide range of electrically powered devices, including not only handheld electronics, such as cell phones, media players, wireless headsets and personal digital assistants, but also higher power devices, such as appliances, tools and electric vehicles. Efficient inductive power transfer has become an area of increasing scientific interest as it can solve some problems associated with traditional wired or contact power transmission. These include but are not limited to corrosion, mechanical friction, clutter and impracticality in places like underwater and subterranean applications. This wireless energy transfer is improved by the optimization of electromagnetic induction, circuit frequency resonance all achieved with advanced power electronics. One of the components of this technology is the precise delivery of the incident electromagnetic fields to the precise location where they are converted to power without broadcasting these fields inefficiently into the surrounding areas. Inductive wireless power transfer systems use electromagnetic fields to transfer power from the power supply to the remote device without the need for wires or any direct electrical contact. Given the nature of electromagnetic fields, many conventional wireless power systems provide improved performance with relatively close alignment between the wireless power supply and the remote device. This has lead to the development of wireless power transfer systems in which the remote device is placed in a specific location or within a relatively small distance from a specific location. For example, it is known to use parallel planar spiral coils in the wireless power supply (e.g. primary) and in the remote device (e.g. secondary) that are concentrically aligned in face-to-face relationship during power transfer. In these types of systems, the primary and secondary are typically of similar size. In some known solutions, the wireless power supply is in a housing, or dock, with a signature surface that forces the portable device to be placed in a specific target location and in a specific orientation. Although providing efficient power transfer, these types of systems lack the desired amount positional freedom that might be desired in some applications.
Although eliminating power/charging cords is by itself a significant and meaningful advantage, wireless power transfer may be even more appealing if the need for close alignment between the wireless power supply and the remote device was reduced or eliminated. From a user perspective, it can be desirable to be able to place a portable device in a random position and a random orientation within the boundaries of a charging surface. It may be even more desirable to the user if the target zone is substantially larger than the secondary device, thus allowing freedom from specific placement and orientation. With this in mind, a number of wireless power transfer systems have been developed to provide increased spatial freedom in aligning the remote device with the wireless power transfer supply. For example, it is known to use a large primary coil to transmit power to one or more smaller secondary coils which are located within the diameter of the large primary coil. Although providing increased spatial freedom, the large primary coil can increase stray electromagnetic fields and dramatically increase parasitic losses. With a large coil within a charging surface, the coil might emit stray electromagnetic field over the entire charging surface. Stray electromagnetic fields can interact with metal within remote device(s) placed on the charging surface, as well as other metal objects that might be placed within sufficient proximity to the wireless power supply. For example, stray electromagnetic fields may cause metal within the remote device to heat, thereby heating the remote device. As another example, stray electromagnetic fields can heat keys, coins, or other metal object placed in proximity to the wireless power supply. To provide some limit on the impact of stray electromagnetic fields, the power supply and/or remote device may have additional magnetic flux guiding materials that are capable of directing the shape of the electromagnetic field. These materials can be arranged to help limit the field from impacting metals within and without the remote devices. As an example, a flux guiding material may be placed between a coil and a battery, or printed circuit board, to reduce/eliminate the impact of the magnetic field on the battery or printed circuit board.
Another conventional option for providing increased spatial freedom is through the use of inductive coils that move behind, under, or above the charging surface to self-align with the portable device. In these solutions, the coils may move automatically by magnetic attraction, or by motorized mechanism, or by manual adjustment or mechanism. These types of system may include relatively complex mechanical and/or electro-mechanical systems that can significantly increase cost and create reliability issues. For example, mechanical assemblies involve moving parts tend to be more likely to fail than purely electronic systems. Systems based on magnetic attraction may have a limited range of movement that will vary with the strength of the attractor magnet and the amount of force required to move the primary. In addition to cost and reliability issues, motorized systems require time for the primary to be moved into the appropriate position. Manually adjusted systems require human intervention and therefore may not be as convenient as they would be if the remote device could be placed randomly within a large zone and forgotten.
In other conventional systems, positional freedom is achieved through the use of arrays of coils behind, under, or above a charging surface. These arrays may include fixed, discrete charging locations, such as a charging pad with two or more primary coils arranged to allow multiple devices to charge side by side. In other embodiments of an array, there may be multiple layers of coils that overlap in a way that allows for less discrete positioning of the secondary device on the charger. Array-type systems require multiple coils and therefore can be more expensive to implement. They may also involve relatively complex controls, such as additional electronic hardware, for determining which coil(s) in the array to energize and for selectively switching the coils to the proper configuration to provide power to the remote device.
The need to meet the ever increasing consumers demand for convenient portable devices is a strong driving force to exploring the potentials of closely coupled inductive power transfer. The basic concepts of this technology have been described in detail in various publications. However, it is believed that inadequate discussion has been directed toward the issue of precise delivery of the magnetic flux for inductive power transfer, as is the case in most transmission pads. Some of the issues can be described as those of spatial freedom (that is, being able to receive power at different locations on a power transfer surface or transmitting pad), and of electromagnetic field broadcast (that is, ensuring magnetic flux is substantially limited to the flux receiving system and does not significantly transfer into the environment). These are areas of some importance to the technology, given the challenges of interference, parasitic heating and regulatory emission limits.
This problem of electromagnetic field broadcast has traditionally been addressed using power electronics to shut down transmission during non-active periods, but even this technique has residual power being broadcast due to the presence of the communication circuits. Furthermore, such a technique including single-coil transmission systems is generally only applicable to small surface area transmission pads, which do not power multiple devices at the same time. In the case of wide surface area charging systems (delivery of power to multiple remote devices), the field broadcast challenges have gone largely undiscussed in literature that addresses multiple receiver transmission. This is because shutting down the circuit at a particular location due to the removal of one remote device will deprive another remote device of power. Conversely, if the electronic algorithm doesn't include shutdown of the system at one location when one or multiple devices are being charged, then a case of magnetic field broadcast into the surroundings can occur, with its adverse effects.